EP1648344B1 - Magnetic resonance-compatible medical implant - Google Patents

Abstract

The invention relates to a medical implant or instrument, particularly a vascular endoprosthesis (stent), comprising a deformable structural part. In order to provide an implant of this type that is magnetic resonance-compatible and can be easily and economically produced, the invention provides that the structural part has a two-layer or multilayer design, whereby layers (2,3) have different electrical and/or magnetic properties. The invention particularly provides that the structural part has a frame structure with openings (5) whereby, in different layers (2,3) of the structural part, the openings (5) are each located at different positions not directly located one above the other.

Description

The invention relates to a medical implant or instrument, in particular a vascular endoprosthesis, with a deformable structural part, which is constructed in two or more layers, wherein the layers have different electrical and / or magnetic properties, wherein the structural part by a metallic interconnected by a plurality of interconnected Struts formed, expandable framework structure, which is designed such that self-contained current paths are avoided within individual layers of the structural part, and wherein the skeleton structure has interruptions, which are located in different layers of the structural member respectively at different, not directly superimposed positions.

Vascular endoprostheses (so-called "stents") and other medical implants or instruments that have a deformable structural part, such. As intravascular filter systems are known from the prior art. In the case of stents, the structural part is usually formed by lattice-shaped metal filaments which are used to support and / or smooth a damaged coronary vessel wall. A stent is - similar to a PTCA treatment - positioned by means of a balloon catheter in the damaged area of the vessel to be treated. Stents are primarily used to prevent acute vascular occlusion or restenosis after PTCA treatment. So-called stent grafts are used to treat aneurysms.

Diagnostic imaging of areas around a stent or similar medical magnetic resonance (MR) implant often proves problematic. This may be due to the fact that the structural part of the implant located in the body of the examined patient consists of paramagnetic material. Due to the magnetic susceptibility of the implant, the magnetic field in the otherwise diamagnetic environment of the implant is distorted, so that artefacts occur in the recorded images. These artifact-related images are then mostly not useful for diagnostic purposes. Medical implants and instruments which have a metallic filament structural part also have the disadvantage that the lattice or net-like structure acts as a Faraday cage in MR imaging, so that the radiofrequency fields irradiated during MR imaging do not enter the MRI Penetrate volume within the implant. Due to this shield, the interior of a conventional stent disadvantageously often remains invisible in MR imaging. This is disadvantageous, in particular, because it is prevented that restenosis in the interior of a stent can be diagnosed early by means of MR imaging.

From the US 2002/0188345 A1 For example, an MR-compatible stent is known whose deformable structural part consists of metal filaments which have interruptions at certain points, so that self-contained current paths within the framework structure are prevented. By preventing eddy currents is achieved so that the stent can not act as a Faraday cage. The radiofrequency fields radiated in MR imaging can thus not induce eddy currents in the stent structure, and MR imaging of the interior of the stent is also possible. A disadvantage, however, in the stent known from the cited US-Offenlegungsschrift that its production is extremely complicated and expensive. In order for the structural integrity of the stent does not suffer, namely, the interruptions attached to prevent eddy currents must be bridged with suitable electrically non-conductive material. For this purpose, adhesives, plastics or the like come into question. In addition, in order for the intermittent metal filaments to have sufficient stability under shear loads, the interruptions in the prior art stent must be additional have a special shape, in the mentioned US 2002/0188345 A1 is described in detail. The preparation of these specially shaped interruptions is particularly complex and technologically difficult. A further disadvantage of the known stent is that no precautions have been taken to minimize the abovementioned susceptibility artifacts in the MR images.

An MR-compatible metallic endoprosthesis of the type mentioned is from the WO 03/015662 A1 previously known. In the previously known endoprosthesis is achieved by combining the manufacturing material with a special design of the framework structure of the structural part that the endoprosthesis in MR images does not cause any artifacts and possibly also an imaging of the lumen of the endoprosthesis by means of MR is possible. The design of the prior art endoprosthesis is such that the struts of the framework structure extend substantially along the longitudinal axis of the endoprosthesis so as not to form closed flow paths in a plane substantially perpendicular to the endoprosthesis longitudinal axis. The previously known prosthesis can have a multilayer structure. For example, a jacket with one or more membranes of polymer material is disclosed. A disadvantage of the previously known endoprosthesis, in turn, especially their comparatively complicated production.

The US 6,231,516 B1 relates to an endoluminal implant connected to a radio frequency coil. Via the radio-frequency coil, electrical energy can be supplied to the implant for therapeutic purposes. The previously known implant may have a multilayer structure, wherein the radio-frequency coil is wound helically around the circumference of a stent. MR-compatible, the prior art implant is disadvantageously not.

An MR-compatible stent is further from the US 6,280,385 B1 known. This stent has a passive resonant circuit with an inductance and a capacitance, wherein the resonant frequency is tuned to the frequency of the MR system used. The structural part of the prior art stent may have a multilayer structure, wherein the different layers have different electrical conductivities. By structuring the Layers the required for the formation of the resonant circuit inductance is generated.

In the article " Reduction of NMR image artifacts by using optimized materials for diagnostic aids and implants "by S. Fritzsche, R. Thull and A. Haase in the journal Biomedical Engineering, Volume 36, Issue 3/1994, pages 42 to 46 For medical implants, the use of materials that are adapted in their magnetic susceptibility to the respective environment is addressed. This is intended to avoid artifacts and signal loss in MR imaging of areas around an implant in the human body.

Another vascular endoprosthesis is from the US 5,667,523 previously known. This document relates to a stent with a radially expandable structural part, which has a multilayer structure. MR-compatible is the prior art stent disadvantageously not.

The WO 2005/013856 A2 , a document falling under Article 54 (3) EPC, relates to a multilayered MR-compatible stent structure, wherein two electrically conductive layers are provided, each interspersed to prevent unwanted formation of closed circuits intermittently inserted into the different layers are each arranged at different, non-superposed positions, whereby a total of a stent high structural integrity is provided, which is inexpensive to produce in comparison to the above-described embodiments of endoprostheses.

Based on this, the object of the present invention is to provide an MR-compatible medical implant or instrument, in particular a stent, with which the MR imaging of the volume within a medical implant or instrument designed according to the invention is particularly well possible.

This object is achieved by the invention on the basis of a medical implant or instrument of the type mentioned at the outset in that the interruptions are arranged in such a way that within at least one of the layers a an end region of the structural part is formed to the opposite end region extending through the current path.

The MR compatibility of the medical implant according to the invention results, on the one hand, from its multilayer structure. Above all, the multi-layer structure has the advantage of a particularly simple and cost-effective producibility of the medical implant according to the invention. The deformable structural part of a stent constructed according to the invention may for example consist of two tube parts with different diameters, which are arranged coaxially with one another. In this case, each individual tube part, as in the case of conventional stents, can be structured, for example by means of a laser, by cutting out a framework structure from a continuous tube in such a way that the two tube parts have the desired different electrical properties. After or structuring of the pipe parts, these can be arranged one inside the other and glued together using a suitable adhesive. Just as well, the pipe parts can first be arranged inside each other and glued together and only then by means of a laser or otherwise structured in the manner according to the invention. Different magnetic properties result from a suitable choice of materials for the two pipe parts.

Similar to conventional stents, the structural part of the medical implant according to the invention has an expandable framework structure formed by a plurality of interconnected metallic struts. So that the desired MR compatibility is ensured, the framework structure, for example by cutting individual struts, interruptions, such that self-contained current paths are avoided within individual layers of the structural part. As a result, eddy currents occurring in the MR imaging are effectively prevented, so that the medical implant according to the invention does not shield the irradiated high-frequency fields. It is essential that the interruptions in different layers of the structural part are each at different, not directly superimposed positions. By having a break in a layer at a position is located in another layer no interruption is provided, the structural integrity of the overall arrangement according to the invention is ensured, which is particularly important for stents, so that they can withstand the radial loads applied by the vessel walls. To ensure adequate structural integrity, according to the invention, a significantly lower production effort is required than is the case with previously known stent structures.

According to the invention, the interruptions are arranged such that, within at least one layer, a continuous current path extending from one end region of the structural part to the opposite end region is formed. This continuous current path can advantageously be helical, so that the individual layers each have the electrical properties of an inductance. It is useful here if the continuous current paths formed within different layers of the structural part are connected to one another, so that the inductances of the individual layers add up to an overall impedance as required in a suitable manner. It is particularly advantageous to connect the continuous current paths of the different layers to one another via at least one electrical capacitance. This results in a total of a resonator structure, which can be used in MR imaging. For this purpose, the resonance frequency of the structure must be matched to the resonance frequency of the MR device. The radiofrequency fields irradiated during MR imaging are then not shielded, as in conventional stents, but - on the contrary - even amplified inside the implant. Thus, the MR imaging of the volume within a stent formed according to the invention is then particularly well possible. The capacity required for the action as a resonator can be formed in a particularly simple manner by means of superimposed, electrically conductive regions of the layers of the structural part of the medical implant according to the invention. Depending on the application, the continuous current paths formed within the individual layers can also be connected to one another via plated-through holes in the end regions of the structural part. By means of such connections, the desired total impedance of the medical implant can be adjusted in a targeted manner. If the largest possible Inductance is sought, it is advantageous if the helically shaped current paths of different layers have opposite directions of rotation, such that add the inductances of the interconnected individual layers and not compensate.

An alternative possibility of generating a resonator structure by means of the implant or instrument according to the invention is to arrange the interruptions in the framework structure such that two or more substantially helical current path sections are formed within at least two superimposed layers of the structural part, the current path sections being different Layers of the structural part are at least partially overlapping each other. Accordingly, each one indicates single layer two or more series-connected helical current path sections, each contributing with their inductance to the total inductance of the resonator. The helical current path sections of different layers partially overlap, so that capacitances are formed in the coverage regions. Overall, this structure is constructed in the manner of a well-known in the field of high-frequency engineering, so-called split-ring resonator. In this case, the series arrangement of two or more helical current path sections, each of which can have a plurality of helix turns, causes the total inductance of the structure to be sufficiently large for the resonance frequency to be within the range of conventional MR resonance frequencies of approximately 20 to 400 MHz.

In order to avoid the mentioned magnetic susceptibility artifacts when using the medical implant according to the invention in MR imaging, it is particularly expedient for at least two of the layers of the structural part to consist of materials having mutually opposite magnetic susceptibilities. Accordingly, at least one of the layers should consist of a diamagnetic material, while at least one further layer consists of a ferromagnetic or paramagnetic material. The opposite susceptibilities largely compensate each other so that the overall effective susceptibility of the implant or instrument is reduced. Distortions of the magnetic fields in the surroundings of the implant then occur only to a reduced extent, and corresponding artifacts in the recorded MR images are reduced.

It is expedient that, in the case of the medical implant according to the invention, the layers of the structural part consisting of electrically conductive material are separated from one another by layers consisting of electrically insulating material. This ensures, above all, that the desired different electrical properties of the individual layers can be specified in a targeted manner. In particular, if targeted interruptions in the framework structure are to prevent the described effect of the structural part of the implant as Faraday cage attached electrical connections between the various layers are prevented, otherwise the interruptions could be bridged and thus ineffective.

The medical implant according to the invention can advantageously be used in an MR imaging method for generating an image of a patient located in the examination volume of an MR device. To suppress magnetic susceptibility artifacts, a paramagnetic contrast agent can be intravenously applied to the patient during imaging, which is composed such that the paramagnetic susceptibility of the blood in the vicinity of the medical implant is substantially equal to the paramagnetic susceptibility of the medical implant itself. Thus, if the environment of the medical implant has substantially the same susceptibility properties as the medical implant itself, local distortions of the magnetic fields do not occur. Accordingly, no artifacts occur in the recorded MR images. Suitable paramagnetic contrast agents may contain at least one substance from the group of ferrites. The now common GdDTPA contrast agent can also be used.

Fig. 1

erfindungsgemäße Gefäßendoprothese im Querschnitt;

Fig. 2

in die Zeichnungsebene abgewickelte Darstellung der unterschiedlichen Strukturierungen der Schichten des Implantats gemäß Fig. 1;

Fig. 3

dreidimensionale Ansicht der Koronarendoprothese gemäß Fig. 1;

Fig. 4

Ersatzschaltbild zur Verdeutlichung der Resonatoreigenschaften des Implantats;

Fig. 5

schematische Darstellung des erfindungsgemäßen erweiterten Split-Ring- Resonators.

Embodiments of the invention are explained below with reference to the figures. Show it:

Fig. 1

Vascular endoprosthesis according to the invention in cross section;

Fig. 2

in the drawing plane developed representation of the different structurings of the layers of the implant according to Fig. 1 ;

Fig. 3

Three-dimensional view of the coronary endoprosthesis according to Fig. 1 ;

Fig. 4

Equivalent circuit diagram to illustrate the resonator properties of the implant;

Fig. 5

schematic representation of the inventive extended split-ring resonator.

The medical implant shown in the figures is a stent that fits into the Fig. 1 and 3 is designated as a whole by the reference numeral 1. The stent 1 has a deformable, expandable structural part with a two-layered construction. An inner layer 2 and an outer layer 3 are arranged one above the other. The layers 2 and 3 of the structural part are formed by two coaxially arranged tubular elements, which in the Fig. 3 are clearly visible. The existing of electrically conductive material layers 2 and 3 are separated by an existing electrically insulating material interlayer 4 from each other. This may be, for example, a layer of adhesive, by means of which the mutually coaxially arranged tubular elements are interconnected.

In the medical implant shown in the figures, the layers 2 and 3, which form the structural part of the stent, have an expandable framework structure formed by a plurality of interconnected metallic struts. This structure is in the Fig. 2 and 3 to recognize. The struts are metal filaments which together form a diamond-shaped grid in the embodiment. The framework structure has interruptions 5, which in the Fig. 2 symbolized by open circles. The Fig. 2 shows only a section of the overall structure of the stent. By the interruptions 5 self-contained current paths within the layers 2 and 3 of the structural part are avoided. Based on the developed in the drawing plane representation according to Fig. 2 It can clearly be seen that in the layers 2 and 3, the interruptions 5 are each at different, not directly superimposed positions. Thereby, as stated above, it is ensured that the structural integrity of the stent 1 as a whole is ensured. Furthermore, the interruptions 5 are arranged such that the tubular layers 2 and 3 each have helical continuous current paths, which is shown in FIGS Fig. 2 and 3 is symbolized by arrows 6. The helical current paths 6 form inductances, which are connected to one another via electrical capacitances are. These capacitances are formed by the superimposed, electrically conductive regions of the layers 2, 3 of the structural part.

Based on the diagram according to Fig. 4 It becomes clear how the inductances and capacitances are interconnected. An inductance 7 is assigned to the inner layer 2 of the stent 1. The coaxial arrangement of the tubular elements of the stent 1 results in capacitances 8, via which the inductance 7 of the inner layer 2 is connected to an inductance 9 of the outer layer 3. Overall, this creates a resonant circuit, the capacitance 8 and the inductances 7 and 9 formed by the current paths 6 being matched to one another in such a way that the resonant frequency is equal to the resonant frequency of an MR device, not illustrated in greater detail in the figures.

The Fig. 5 schematically shows the split-ring structure described above, which can be achieved according to the invention by suitable arrangement of the interruptions within the framework structure of the implant. In the illustration according to Fig. 5 The solid lines symbolize current domain sections 6 'within the outer layer 3 of the resonator, while the dashed lines represent current domain sections 6 "formed by appropriate placement of the breaks within the layer 2. The current path sections 6' and 6" are each helical in shape two full helix turns each. In the presentation of the Fig. 5 vertically aligned lines in the top view symbolize the entire structure of the rearmost sections of the helical turns, while the oblique lines represent the frontmost sections of the helix. In the Fig. 5 4, four current path sections 6 'and 6 "are arranged one behind the other in layers 2 and 3, the individual current path sections being separated from each other by insulating sections 10. The arrangement is such that an insulating section 10 of the inner This results in a total of the at least partially overlapping arrangement of the current path sections, thus creating an extended split-ring structure which can be used as an MR resonator in the manner described above is.

Claims (12)

1. A medical implant or instrument, in particular a vascular endoprosthesis (1), comprising a deformable structural part which is of a two-layer or multi-layer structure, wherein the layers (2, 3) have different electrical and/or magnetic properties, wherein the structural part has an expandable frame structure which is formed by a plurality of interconnected metallic struts and which is so designed that current paths which are closed in themselves within individual layers (2, 3) of the structural part are avoided, and wherein the frame structure has interruptions disposed in different layers (2, 3) of the structural part at respective different positions which are not in directly mutually superposed relationship, wherein the interruptions (5) are so arranged that within at least one layer (2, 3) there is formed a continuous current path (6) extending from an end region of the structural part to the opposite end region.
2. A medical implant or instrument according to claim 1 characterised in that the continuous current path (6) is of a helical configuration.
3. A medical implant or instrument according to claim 1 characterised in that the interruptions (5) are so arranged that within at least two mutually superposed layers (2, 3) two or more substantially helical current path portions (6', 6") are formed, wherein the current path portions (6', 6") of different layers (2, 3) of the structural part are arranged in at least partially mutually overlapping relationship.
4. A medical implant or instrument according to one of claims 1 to 3 characterised in that the continuous current paths (6) or current path portions (6', 6") formed within different layers (2, 3) of the structural part are connected together.
5. A medical implant or instrument according to claim 4 characterised in that the current paths (6) or current path portions (6', 6") are connected together by way of at least one electrical capacitor (8).
6. A medical implant or instrument according to claim 5 characterised in that the capacitor (8) is formed by mutually superposed, electrically conducting regions of the layers (2, 3) of the structural part.
7. A medical implant or instrument according to one of claims 1 to 7 characterised in that the current paths (6) or current path portions (6', 6") are connected together by way of through-contacting means between the layers (2, 3).
8. A medical implant or instrument according to claim 2 or claim 3 characterised in that the helical current paths (6) or current path portions (6', 6") of different layers (2, 3) are of opposite directions of rotation.
9. A medical implant or instrument according to claim 5 characterised in that the capacitor (8) and the inductances (7, 9) formed by the current paths (6) or current path portions (6', 6") are matched to each other in such a way that a high frequency generator is formed, the resonance frequency of which is equal to the resonance frequency of an MR apparatus.
10. A medical implant or instrument according to one of claims 1 to 9 characterised in that at least two of the layers (2, 3) of the structural part comprise metals of mutually opposite magnetic susceptibilities.
11. A medical implant or instrument according to one of claims 1 to 10 characterised in that the layers (2, 3) of the structural part are formed by two or more coaxially arranged tubular elements.
12. A medical implant or instrument according to one or more of claims 1 to 11 characterised in that layers (2, 3) of the structural part, that comprise electrically conducting material, are separated from each other by intermediate layers (4) comprising electrically insulating material.

Images